In the case of a conventional conditional access system for reception of digital broadcasting, a broadcasting station (transmitter side) encrypts a program (content) and only limited receivers (receiver side) are allowed to reproduce the program. The receivers are generally required to have a receiving apparatus that decrypts, in real-time, the broadcasted encrypted content.
The conventional conditional access system for reception will be described with reference to FIG. 12. Japanese satellite digital broadcasting uses a block encryption method to scramble a program (content). As is shown in FIG. 12, in the block encryption method, content is scrambled (encrypted) using a scrambling key (Ks) that is changed in a short term (for example, about one second). Content information about the scrambling key and the content is encrypted using a work key (Kw) that is changed in a long term (for example, about a half year to a year) as an Entitlement Control Message (ECM) (encryption related information). The work key (Kw) and contractual contents of each receiving apparatus are encrypted using a master key (Km) that is commonly retained by the broadcasting apparatus of the transmitter side and the receiving apparatus of the receiver side as an Entitlement Management Message (EMM) (individual related information, including the encrypted work key). The ECM and the EMM are multiplexed with the encrypted content and transmitted together.
At the receiver side in which this multiplexed broadcasting is received, the multiplexed broadcasting is separated to EMM (the entitlement management message including the encrypted work key), ECM (the entitlement control message including the encrypted scrambling key), and the encrypted content. The work key is obtained by decrypting the EMM using the common master key retained by the broadcasting apparatus of the transmitter side. The scrambling key is obtained by decrypting the ECM using the work key. The content is obtained by decrypting the encrypted content using the scrambling key. The decrypted content is stored, if necessary, in a storage device.
Thus, the content is scrambled using the scrambling key, only the limited receivers who are authorized by the broadcasting station (broadcaster) and able to obtain this scrambling key can descramble the scrambled content and reproduce the content. Accordingly, it is not easy for an unauthorized receiver who has no subscription contract with the broadcasting station (broadcaster) and has no specific receiving apparatus, to descramble and reproduce the content. The conventional reception system prohibits an unauthorized receiver from reproducing the content, which enhances the security of the broadcasted content.
The principle of the conditional access system for reception has been described above. The conditional access system for reception includes, for example, a system based on the standard specification “conditional access system for reception for digital broadcasting” (ARIB STD-B25) defined by the Association of Radio Industries and Businesses (ARIB) of Japan.
Thanks to recent increases in the memory capacity of storage devices in receiving apparatuses and recent improvements in user interfaces, more receivers are storing all contents broadcasted by a broadcasting station for tens of hours and afterwards selecting contents as desired instead of storing only selected contents.
In this case, if a pay content is included in the stored contents, the receiver needs to descramble the scramble on the pay content before reproducing the pay content.
The broadcasting station needs to appropriately charge the receiver who reproduces the pay content, while the receiver who reproduces the pay content should be reasonably charged. However, the receiver stores the pay content but sometimes does not reproduce the stored pay content. The receiver would prefer to be charged when the receiver reproduces the stored pay content. In the case of the conventional conditional access system for reception, however, the receiver is charged once they store the descrambled content even if they do not reproduce the descrambled content. Accordingly, a method of descrambling the content when the content is retrieved from the storage device is desired.
The encrypted digital content is usually stored according to the content or the file. The encrypted content can be descrambled according to the content or the file efficiently. The conventional descramble processing, however, is executed by the stream, which degrades the efficiency of the descrambling process.
Furthermore, descrambling by content or file requires many more keys than descrambling by stream, and appropriately controlling so many keys is difficult.
Accordingly, it is an object of the present invention to provide a content transmission apparatus and method, a content reception apparatus and method, a content transmission program, and a content reception program that can eliminate one or more of the above problems by descrambling the scrambled content efficiently, making the control of encryption keys easy to protect the copyright of the content.